Xerography has become one of the most important everyday events in today's office environment. A xerographic process, which allows high quality permanent images to be produced from xerographic devices such as copiers and laser printers, comprises a sequence of steps which include: (1) charging, i.e., causing a photoconductor to become charged; (2) forming electrostatic latent images on the photoconductor upon exposure to light (i.e., corona discharge); (3) using a toner to develop positive images on the photoconductor; (4) transferring the positive images from the photoconductor to a print medium, which can be a plain paper or a transparent film; (5) fusing, i.e., fixing the positive images on the print medium; (6) cleaning the remaining toners from the photoconductor; and (7) erasing electric charges from the photoconductor. From these many functions that a photoconductor is involved in a xerographic process, there is no doubt that the photoconductor is the nerve center of a xerographic device, just like what a heart is with respect to a human body.
Photoconductors can be classified according to their constituent materials as either an inorganic or an organic photoconductor (OPC). Traditionally, the photoconductors that have been used in copiers such as selenium (Se), cadium sulfide (CdS), non-crystalline silica (.alpha.-Si), etc., belong to the class of inorganic photoconductors. Inorganic photoconductors have the advantages of high sensitivity, high hardness, high abrasion-resistance, and can be used for making hundred of thousands prints with little or no degradation in print quality. However, inorganic photoconductors also present many disadvantages such as the high manufacturing cost and the relatively difficult quality control, etc.
On comparison, organic photoconductors, which can be more easily and relatively inexpensively manufactured, have gradually replaced inorganic photoconductors as the main stream material in the market for use with laser printers and certain copiers. Organic photoconductors also have the advantages of having low or no toxicity and thus do not cause pollution problems, and can produce sharp images. However, organic photoconductors have often been recognized as having the shortcomings of lacking the same light and heat stability as inorganic photoconductors, and are of relatively shorter service life. Due to these weaknesses, organic photoconductors are largely limited in their use to low to medium speed copiers.
As organic photoconductor is an insulator when it is not exposed to light. After exposure to a light source, the incident photons from the light source are absorbed, resulting in a charge separation which causes electron-hole pairs to be formed. Under the influence of an externally applied electric field, the electrons and holes so formed will move in opposite directions, thus enabling the organic photoconductor to become an electric conductor. One of the key elements of the photoconductors is that, when charges are generated upon exposure to light, the electric charges are maintained on their surface after the incident light is terminated. The ability to prevent the charges to be quickly neutralized is one of the important characteristics required of a good organic photoconductor (or of any photoconductor, organic or inorganic). An organic photoconductor also provides the required structure to conduct the electric charges.
While organic photoconductors can be classified, according to their development history, as belonging to either the single-layer type or the functionally-separated multi-layer type, the functionally-separated multi-layer types are of the predominant type. A functionally-separated photoconductor comprises a charge generation layer (CGL) and a charge transport layer (CTL). The two layers provide the separate but cooperating functions such that when the charge generating layer is exposed to light, electron-hole pairs will be generated therein. And the charge transport layer causes the generated charges to be transported to the surface of the photoconductor.
The charge generation layer typically contains a charge generation material (CGM), such as phthalocyanine pigments, azo pigments, etc., uniformly dispersed in a polymeric binder. The charge generating material is provided to absorb the incident light and produce resultant charges. In order to provide adequate light absorption, the thickness of the charge generating layer is typically designed to be between 0.1 and 0.3 .mu.m. Similarly, the charge transport layer typically contains a charge transport material (CTM), such as triphenylamine, etc., dissolved in a polymeric binder. The functionality of the charge transport layer is provided by the small organic molecules (i.e., the charge transport materials contained therein), while the polymeric binder provides the required filmability, insulation and mechanical strengths, etc. On the one hand, the charge transport layer must have an adequate thickness so as to provide the required mechanical strength. On the other hand, its thickness must not be too large so as to impede the speed of charge transport. Typically, the thickness of the charge transport layer is provided which has a thickness of about 10 to 30 .mu.m.
The transportability of an organic photoconductor, i.e., the speed at which charges can be transported in a charge transport layer, is determined primarily by two factors: (1) the compatibility between the charge transport material and the polymeric binder, the charge transport material must be soluble in the polymer binder; and (2) concentration of the charge transport material in the polymer binder. In order to increase the transportability of the charge transport layer, the concentration of the charge transport material must be relatively high, so as to reduce the intermolecular distance therebetween. However, a higher concentration of the charge transport material would inevitably reduce the mechanical strength of the charge transport layer and decrease the service life of the organic photoconductor made therefrom. Furthermore, in order to satisfy the first requirement stated above and maintain a manageable cost structure, the polymer binders are typically selected from a limited number of well-known commercially available thermoplastic resins such as polycarbonate, polystyrene, poly(methyl methacrylate) (PMMA). These thermoplastic resins have limited mechanical strength and relatively low hardness. Thus, there are practical limits, under the current constraints, within which the service life of the organic photoconductor can be extended.
In U.S. Pat. No. 4,489,148, the content thereof is incorporated by reference, it is disclosed an improved photoconductive device comprised of a substrate, an adhesive layer, a hole transport layer, an inorganic panchromatic layer, an organic photoconductive layer sensitive to infrared radiation, an inorganic photogenerating layer, and a polymeric overcoating layer. The organic photoconductive layer is selected from the group consisting of organic photoconductive compositions, charge transfer complex compositions, dye sensitizers, or mixtures thereof. The hole transport layer contains hole-transporting materials dissolved in transparent resinous material such as polycarbonates, polyesters, phenoxys, etc. These polymers do not provided observable improved mechanical strength.
In U.S. Pat. No. 4,923,775, the content thereof is incorporated by reference, it is disclosed an improved electrophotographic imaging member comprising a supporting substrate, at least on photoconductive layer and an overcoating layer comprising a polymerized silane. The polymerized silane comprises a reaction product of hydrolyzed alkoxy silane. The overcoating layer overlies a charge transport layer, which comprises a diamine dispersed in a polycarbonate resin.
In U.S. Pat. No. 5,166,021, the content thereof is incorporated by reference, it is disclosed improved layered photoresponsive or photoconductor imaging members containing a protective polycarbonatefluorosiloxane polymer overcoating. The imaging members contain a hole transport layer with a polycarbonate resin binder. One of the shortcomings of the organic photoconductor disclosed in the '021 patent was that it did not provide sufficient abrasion resistance or surface hardness.
In U.S. Pat. No. 5,270,139, the content thereof is incorporated by reference, it is disclosed an improved photoconductive device comprising a conductive substrate, a charge generation layer and a charge transport layer. The charge transport layer contains a charge transport material dissolved in a copolymer of styrene and methyl methacrylate.
Organic photoconductors offer many strong advantages such as lowered manufacturing cost, high mass-producibility (via a variety of available coating techniques), low pollution, and flexibility of molecular design to tailor for a specific application, over their inorganic counterparts. However, the usage of the organic photoconductors has been hampered primarily by their relatively inferior photosensitivity, inadequate mechanical strength, and relatively short service life. As the issue relating to environmental pollution has become an increasingly important concern, it is highly desirable to expend research and development effort so that we can further improve the properties of organic photoconductors such that they can satisfactorily replace inorganic photoconductors and eliminate or substantially minimize a potential pollution stream from entering our prescious and increasingly volunerable environment.